An electrographic recording apparatus forms an image by depositing spots of electric charge from styli in a writing head onto an electrographic recording receiver, and developing the latent electrostatic image with a liquid or dry electrostatic toner. A continuous sheet of receiver is drawn over the writing head, which contains closely packed styli that extend across the width of the image area of the receiver. Each stylus can deposit a separate spot of electric charge on the receiver. See, for example, U.S. Pat. No. 3,657,005, issued Apr. 18, 1972.
The receiver comprises a dielectric layer coated on a substrate which has a conductive surface in electrical contact with the dielectric layer. In order for the recording process to operate properly, it is necessary that the air gap between the styli and the exposed surface of the dielectric layer be kept within well-defined limits. This can be accomplished by recessing the styli in the writing head and letting the writing head ride directly on the dielectric layer, but that approach may damage the dielectric layer.
The correct distance between the styli and the dielectric layer can also be maintained by embedding in the dielectric layer insulating particles which extend above the surface of the dielectric layer and function as spacer means. The writing head is kept in contact with these particles as the receiver is drawn across it, thereby maintaining a uniform and correct distance between the styli and the dielectric layer. Charges are deposited on the dielectric layer when a sufficient voltage is applied between a stylus and the conductive surface of the substrate to break down the air between the stylus and the dielectric layer.
The substrate is either entirely conductive or it comprises a conductive layer on an insulating support. IN order for a charge to be deposited on the dielectric layer, the conductive surface of the substrate must be grounded. Substrates that are entirely conductive can be easily grounded through the exposed (opposite) surface of the substrate. However, this invention is concerned with receivers that have a substrate which comprises a conductive layer on an insulating support and, therefore cannot be grounded through the exposed (insulating) surface of the substrate. These receivers are normally grounded by means of conductive stripes along the edges of the receiver which are in electrical contact with the conductive layer. While this method of grounding is satisfactory for a receiver that is not very wide, if the receiver is wide the resistance between these stripes and a stylus at the center of the receiver is greater than the resistance between a stylus near the edge of the receiver and the closest stripe. This variation in resistance may result in corresponding non-uniform image quality. If the resistance is too high, the charge builds up on the conductive layer because it cannot be drained away fast enough, which results in toner being deposited on the receiver during development where it is not intended. Also, a loss in image density can occur because the air breakdown between the stylus and the dielectric layer is inhibited. On the other hand, if the resistance is too low, charge is not easily placed on the receiver and there is a loss in image density.
In addition, grounding the receiver by means of edge stripes complicates the manufacturing process because a procedure must be provided for applying these stripes to the receiver. It also complicates inventory management because, in order to minimize the cost of making the receiver, the stripes are applied to the receiver at the same time the receiver is made. Since the receiver is sold in many different widths and the receiver cannot be longitudinally cut once the edges stripes are in place, it is necessary to keep an inventory of each width, which significantly adds to inventory cost.